The presence of particulate matter, such as soot particles, in the environment has brought about an increased interest in the development of methods and devices for the determination of particulate concentration. The emission of soot from engines, power generation facilities, incinerators, or furnaces, for example, represents a loss of useful energy and further is a serious environmental pollutant and a health risk. However, the presence of soot in flames can also have positive effects. For example, the energy transfer from a combustion process is largely facilitated by the radiative heat transfer from soot. Thus, to understand soot formation and develop control strategies for soot emission or formation, measurements of soot concentrations are necessary. Laser Induced Incandescence (LII) is a good diagnostic tool for measurements of particulates as the LII signal is proportional to a particle volume fraction.
According to Planck's law all objects emit electromagnetic radiation. This radiation is invisible to the unaided eye for temperatures below 900.degree. K. However, if an object is heated to temperatures exceeding 3000.degree. K., the emitted light intensity of all visible wavelengths is sufficient to make the object appear white-hot, i.e. incandescence occurs. The intensity of the electromagnetic radiation increases with the temperature of the object and the peak wavelength of the emission shifts towards shorter wavelengths. In laser induced incandescence (LII) a volume of gas containing particulate matter, e.g. soot, is exposed to a pulsed high-intensity laser light source. The particulate matter or particles absorb laser light energy, heating to temperatures far above the surrounding medium. At these elevated temperatures, for example at about 4000.degree.-4500.degree. K. in the case of soot, the particles incandesce strongly throughout the visible and near infrared region of the light spectrum. The maximum particle temperature is controlled by the point at which particle evaporation becomes the predominant heat loss mechanism. -Any further increase in laser light energy then tends to result in an increase in the evaporation rate rather than an increase in particle temperature. In accordance with Planck's radiation law, the radiative emission at these elevated temperatures increases greatly in intensity and shifts to blue light wavelengths as compared with the non-laser heated particle and flame gases. Thus the LII incandescence signal is readily isolated from natural flame emission. Because of the rapid time scale and good spatial resolution, as well as its large dynamic range, LII is well suited as an optical diagnostic to measure particle volume fraction in turbulent, i.e. time-varying, combustion and practical devices. The technique provides high temporal and spatial resolution not provided by previous methods.
At present there is a need for real-time airborne particulate concentration measurements and for soot measurements in turbulent combustion environments. In addition spatially resolved measurements are needed.
In order to measure particulate matter in a turbulent flame the following requirements have to be met: good spatial resolution, good temporal resolution, discrimination against flame radiation, and a large dynamic range. Turbulent flames are found in most practical combustors, such as gasoline engines, Diesel engines, gas turbine engines, furnaces, and boilers, and the control of emitted particles is required to reduce health risks.
Current methods for measuring diesel particulates are the Bosch Smoke Number and the direct mass sampling. In the Bosch Smoke Number methods particulates are collected on filter paper from a portion of the exhaust stream and the light transmission through collected sample is measured. This is compared against a calibration chart to determine the particle mass flow. This method has a poor time and spatial resolution. The direct mass sampling method is the official method of the EPA and measures the mass of soot from a difference of the mass of the soot on a filter and subtracting the mass of the filter. This method, however, has a limited accuracy, particularly for low emission vehicles. Both methods suffer a loss in accuracy when the source of the emitted particles produces lower emissions and thus require significantly longer testing for low emission combustors.
LII can fill the need for particulate measurements since the LII signal is proportional to a particulate volume fraction over a wide dynamic range. However, LII provides a relative measure of particulate concentrations and requires a calibration for quantification of particulate concentrations. Currently, calibration of the technique for absolute particulate concentrations may be made by in situ comparison of the LII signal to a system with a known particle volume fraction determined through traditional methods. Using this empirical calibration procedure LII has been used to measure particle volume fraction in steady-state and time-varying diffusion flames, premixed flames and within engines and in engine exhaust streams.
It is an object of the present invention to perform absolute light intensity measurements in LII and thus avoid the need for a calibration in a source of particulates with a known concentration. It is a further object of the invention to determine a particle volume fraction using absolute light intensity measurements in LII. This requires a knowledge of the particle temperature either from a numerical model of particulate heating or experimental observation of the particulate temperature.
It is an object of the invention to provide a method and an apparatus which are calibration independent for measuring particulate concentrations, i.e. they do not require a source of particulate of known concentration and particle type.
Further, the development of absolute light intensity measurements prepares the basis for providing portable LII instruments. This is particularly useful for applications of exhaust particulate measurements in engine test cells in laboratories, emissions compliance measurements and road-side checks, for applications of stack particulate measurements in furnaces and boilers, for airborne particulate monitoring, and for on-line process monitoring, where calibration in a source of particulates with a known concentration may be impractical.